It is often necessary to have to lock a cylindrical shaft in use position relative to another element. This is particularly the case when using a jack or a press.
Jacks or presses are used when it is necessary to exert a force, for example of pressure, that the user alone cannot exert for sufficient time or that he cannot exert at all.
Hydraulic jacks are thus used in numerous fields, and, for example, the field of conveying for elevators, lifts, rockers, positioning actuators, etc.; the field of public works and of shipyards for the positioning of drilling platforms, maneuvering and blocking of lock gates; the field of transportation for barges, dredges, boxcars and special carriers, bridges, . . . .
In these applications, it is often necessary that the jack be kept in position regardless of its load for rather long periods.
To hold, i.e., to block, the jack in a given position, the jacks are equipped with locking means.
Today, various locking means are used. The most well known is a hydraulic locking means, using a hydraulic fluid such as a hydraulic oil. However, this type of hydraulic means does not make it possible to hold a jack in position under a load for a long period. Actually, it is very difficult to maintain the position due to the change in volume of the oil by compression or expansion as a result of temperature change. Over the longer term, these systems can also have leaks that no longer allow a stable maintaining of position.
To overcome these problems, other types of blocking systems for hydraulic jacks are used.
One of these blocking systems is a locking system with an element of the sleeve type. This system consists of a conical housing in which a ring or sleeve of complementary conical exterior shape is placed. It is in this ring that the piston rod of the jack slides. The ring can be moved axially in relation to the housing to clamp the piston of the jack. Movement is achieved using a piston placed in the narrow part of the housing. This piston moves under the pressure of a hydraulic fluid or any other suitable fluid. When it is under pressure, the piston moves the ring in a direction opposite to that of the load of the piston of the jack, and makes it possible for the piston rod of the jack to slide. When it is no longer under pressure, the piston moves the ring in the same direction as the load of the piston of the jack, which by constricting around the piston of the jack prevents the latter from sliding. This system is, for example, illustrated in the patent EP2226513.
A drawback of this type of system comes from the fact that the blocking can take place only in a single sliding direction of the piston of the jack. It is therefore necessary that the hydraulic jack be used vertically.
Another blocking system used and described in the patent EP0534879 relates to a double-action jack. This system is characterized by the fact that the piston rod is mounted tight in a cylinder, said piston rod being immobilized in the cylinder because of a lack of pressure from a fluid and freed in movements relative to this cylinder by an expansion provided by the pressure of a fluid between the piston and the cylinder.
A drawback of this system comes from the fact that the tight mounting of the piston in the cylinder makes it a difficult system to assemble.
Another significant drawback of these systems is that in case of failure in the area of the hydraulic fluid, the system is held in blocked position, which poses problems in terms of safety.